The present invention relates to a semiconductor device including a capacitor having a three-dimensional structure that allows high integration.
In pace with the recent development of digital technology and the need of processing and storing large quantity of digital data, the development of high functioning electronic equipments and the downsizing of semiconductor devices have also significantly accelerated. Accordingly, in order to achieve a highly integrated dynamic RAM, research and development on a novel method of using a high dielectric material as a substitute of the conventional silicon oxide or silicon nitride in a capacitive insulating film have been widely undertaken.
In addition, research and development related to ferroelectric films having spontaneous polarization property, and aiming at producing a practical and non volatile RAM that can operates in a low operational voltage and at a high reading and writing speed have also been carried out extensively.
In order for a capacitor, in which an insulating metal oxide composed of high dielectric material, ferroelectric material or the like is used in a capacitive insulating film, to be applicable to a highly integrated mega-bite memory, it is required of the capacitor to have a small surface area and yet a three-dimensional structure having a large capacitance.
A conventional capacitor will be described with reference to a drawing as follows.
FIG. 7 illustrates a cross-sectional structure of the main part of a conventional capacitor (see for example, U.S. Pat. No. 5,877,062).
As shown in FIG. 7, an interlayer dielectric film 101 is formed on a semiconductor substrate 100, and a contact plug 102 composed of polysilicon is formed within the interlayer dielectric film 101. Next, an oxygen diffusion barrier layer 103 composed of patterned conductive nitride is formed over the interlayer dielectric film 101 and the contact plug 102, and a lower electrode 104 is formed thereon. The oxygen diffusion barrier layer 103 is composed of a material such as titanium nitride (TiN), titanium aluminum nitride (TiAlN), titanium silicon nitride (TiSiN), tantalum silicon nitride (TaSiN), tantalum aluminum nitride (TaAlN), or tungsten nitride (WN), and the lower electrode 104 is composed of a material such ruthenium (Ru), iridium (Ir), platinum (Pt), osmium (Os), palladium (Pd), tungsten (W), molybdenum (Mo) or cobalt (Co).
The oxygen diffusion barrier layer 103 and the lower electrode 104 are completely covered by a protective layer 105, such that the protective layer 105 is in contact with the sides of the oxygen diffusion barrier layer 103 and the sides and top surface of the lower electrode 104. The protective layer 105 is selectively formed on the sides of the oxygen diffusion barrier layer 103 and the sides and top surface of the lower electrode 104 by electroplating, and is composed of a material such as ruthenium (Ru), iridium (Ir), platinum (Pt), osmium (Os), tungsten (W), molybdenum (Mo), cobalt (Co), nickel (Ni), gold (Au) or silver (Ag). A capacitive insulating film 106 composed of a high dielectric or ferroelectric material is formed covering the protective layer 105, and an upper electrode 107 is then formed covering the capacitive insulating film 106.
According to reference 1, crystallization of the capacitive insulating film 106 is performed by a heat treatment in an oxygen ambient and under a temperature between 500° C. and 800° C. inclusively. The protective layer 105 has a function of preventing oxygen atoms from diffusing into the inner portion of the oxygen diffusion barrier layer 103 from the sides, and thus an increase in resistance caused by oxidation of the surface of the contact plug 102 can be prevented.
After substantial study and research, the applicants of the present invention arrived at a conclusion that it is not possible to fabricate a highly integrated mega-bite memory from a conventional capacitor provided with the protective layer 105, which prevents oxidation of the contact plug 102. The reasons are as follows.
From experiments, the applicants discovered that among the materials used for composing the protective layer 105 that protects the contact plug 102 from oxidation, only iridium (Ir) is capable of preventing the diffusion of oxygen during the heat treatment, which is performed under a temperature between 500° C. and 800° C. inclusively, for crystallizing the capacitive insulating film 106. Moreover, in order to prevent the sides of the oxygen diffusion barrier layer 103 from being oxidized, the thickness of the iridium has to be 100 nm or more. In other words, if the protective layer 105 is provided in a conventional capacitor, the size of the capacitor in the horizontal direction will increase by 200 nm (two layers of 100 nm) with respect to the surface of the substrate.
In order to achieve a highly integrated mega-bite memory, the size of the lower electrode in the horizontal direction cannot be more than 500 nm. However, if the protective layer 105 having a thickness of at least 100 nm is provided in a conventional capacitor, the size of the lower electrode 104 in the horizontal direction, in addition with the two layers of the protective layer 105 measuring at least 200 nm, becomes at least 700 nm. As a result, since each capacitor is 1.4 times larger than the desirable design dimension, a low cost production of highly integrated memory cannot be achieved. Hence, a highly integrated mega-bite memory cannot be produced from a conventional capacitor.
In addition, since the protective layer 105 is formed by electroplating, materials other than metals cannot be used.